This invention relates to compositions and methods of waste treatment.
This invention has particular but not exclusive application to compositions and methods for immobilizing heavy metal and metalloid wastes in a form suitable for geological disposal, and for illustrative purposes reference will be made to such application. However, it is to be understood that this invention could be used in other applications, such as stabilization of other metallic waste streams.
Metal and metalloid waste streams often contain metal salts which are highly toxic or form toxic oxyanions. For example, arsenic containing waste streams present a significant toxic hazard in land fill or the like due to the high leachability of arsenates (III) and the III and V state oxides of arsenic. In the past it has been proposed to immobilize arsenic containing waste materials by encapsulation in portland cement. The arsenic oxides thus encapsulated are of variable oxidation state and generally comprise a mixture of compounds with arsenic in the III and V states. It has been proposed to treat the waste stream with H.sub.2 O.sub.2 or other oxidizing agents to oxidize arsenic III to less soluble and less toxic arsenic V.
A typical waste stream containing high levels of arsenic is the waste produced by metal extraction from ores or concentrates of arsenopyrites. In general, condensed arsenic oxides are produced by pyrometallurgical processes and are precipitated from a condensed gas stream. It has been proposed to mix the oxide directly with aqueous portland cement slurry and cure the mixture in (generally) mild steel containment or directly into fill or into other containment. The disadvantages of this process relates to the instability of the compositions to certain conditions encountered in land fill disposal, including susceptibility to leaching at low and high pH, and poor mechanical properties. Such compositions typically have compressive strengths of about 10 MPa at an equivalent As.sub.2 O.sub.3 loading of 50 wt % compared with a strength of 20 MPa for portland cement per se when mixed with an equivalent amount of clean building sand. Where the aforedescribed H.sub.2 O.sub.2 oxidation process is used to reduce the solubility of %he incorporated waste, the process becomes uneconomic.
Other prior art proposed methods for arsenic waste stream treatment include mixing arsenic III-V oxides with an excess, generally between 3000 to 5000 mol %, of slaked lime Ca(OH).sub.2 in water slurry to form generally insoluble calcium arsenate (III) and calcium arsenate (V) precipitates having a general formula Ca.sub.x (AsO.sub.4).sub.n. However, the precipitates have proven to be of variable leachability, probably due to the presence of a proportion of soluble compounds including arsenic III compounds. Also, the compositions are acid labile particularly with respect to CO.sub.2 or carbonates in the presence of groundwater or water vapour.
Additionally, acidic arsenic III-V oxide containing waste streams have been mixed with 200 or more mole % of ferric (oxy hydroxide), then adjusted to pH 5 to pH 8 with slaked lime in water to form ferric oxyarsenic compounds comprising mixed arsenate (III) and arsenate (V). Again these compositions are acid labile although less than Ca.sub.x (AsO.sub.4).sub.n, and are subject to leaching in the presence of carbonate and/or bicarbonate and water.
Both of the foregoing processes are only useful for treatment of waste streams containing metals or metalloids forming oxyanions such as arsenic.
In general the prior art processes do not produce a "volume reduced" waste stream, that is, the prior art processes do not generally result in relatively dense stabilised waste products incorporating the waste species high mole percentages by weight. In addition, they do form thermodynamically stable mineral structures.
The existence of proportions of volatile arsenic compounds in arsenic waste streams has generally prevented the use of calcining processes for incorporation of such wastes in thermodynamically stable mineral structures.
The present invention aims to substantially alleviate least one of the above disadvantages and to provide compositions and methods of waste treatment which will be reliable and efficient in use. Other objects and advantages of this invention will hereinafter become apparent.